Modern data communication systems commonly require interchange of data between a computer and a remote computer or terminal device. This data interchange may take place over voice grade telephone lines that are either leased from common carriers or are a part of the dial-up telephone network. In either event, modems are required to convert the baseband data signals from the computers or terminals to voice frequency signals suitable for transmission over the telephone lines and to convert the voice frequency signals on the telephone line to baseband data signals suitable for application to the computer or terminal.
Known modems (or data sets) employ different conversion schemes, frequency shift keyed (FSK) modulation and differential phase shift keyed (DPSK) modulation comprising two of the more popular schemes. Bell System data set types 103, 108, 113, 202 and 212 employ one or the other or both of these modulation techniques. The data sets have hard-wired circuitry to perform the data signal conversion (modulation and demodulation) functions.
Another function of data sets, such as the above mentioned Bell System data set types, is to provide communication line control. Communication line control involves the interchange of control and status signals and instructions between the terminal or computer, the data set and the telephone line, which interchange follows certain fixed or predefined procedural sequences in accordance with the "mode" of the data set, the "status" of the telephone line and instructions from the terminal or computer. For example, assuming that the terminal operator or the computer wishes to originate a call through the common carrier switched network, an appropriate instruction (or signal) is sent to the data set which assumes an "originate" mode and goes "off-hook" by completing a path across the tip and ring leads of the dial-up telephone line. The switch network returns "dial tone" which is, optionally, recognized by the terminal operator or by an accompanying automatic calling unit which, in turn, might pass a "proceed-to-dial" indication to the computer or terminal. If, when the data set is in an "idle mode", there is an incoming call, the data set identifies the "ringing" on the telephone line and advises the terminal that a remote set is calling. Assuming that the terminal returns a "data terminal ready" instruction, the data set assumes an "answer" mode, goes "off-hook" and returns an answering tone to the telephone line (and then "looks" for a response from the originating set). During the call, either set, originating or answering, looks for call termination instructions from the terminal and for "disconnect" by the remote set, the latter being identified by "loss of carrier" on the telephone line. In either event, the set assumes the "idle mode" and goes "on-hook" to terminate the telephone connection. These communication line control functions are performed by hard-wired circuitry in the data sets.
Another procedural sequence, which could be characterized as communication line control, constitutes the obtaining of dialing digits from the terminal (when the set assumes the originating mode) and the sending of corresponding multifrequency dialing signals to the telephone line. Bell System Automatic Calling Unit type 801 performs this line control sequence utilizing hard-wired circuitry.
Recent advances in digital processors have resulted in the development of signal processors for converting baseband signals to voiceband and voiceband signals to baseband. The digital signal processor is controlled by software instructions which converts baseband signal to voiceband by, for example, digitally synthesizing a sinusoidal wave in response to the baseband input signal and by computing a baseband signal in response to a digitally synthesized wave derived from voiceband signals.
Moreover, the instruction set could define algorithms for various types of signal conversions, such as FSK or DPSK modulation or conversion of digital signals to multifrequency tones, the digital signal processor being capable of executing the various mathematical computations required by the algorithms and therefore the flexibility to execute the signal conversion functions of varius modems. This flexibility is described in U.S. Pat. No. 3,958,191 issued to G. D. Jones, Jr. which discloses the utilization of three instruction sets for a digital signal processor to perform, alternatively, FSK, DPSK or multifrequency modulation.
It is an object of this invention to utilize digital processors for executing communication line control. More specifically, it is an object of this invention to provide a processor based data set having the flexibility to execute various signal conversion and line signal control algorithms appropriate for different type of data sets or modems.